A large-scale storage device that manages and retains electronic mails and also manages and retains materials and documents in the form of electronic data is in great demand in order to promote computerization of business and to cope with regal regulations. As an existing large-scale storage device, a storage device of the type that a hard disk which is excellent in quick-responsiveness and is capable of handling mass data is installed is widely used.
In addition, recently, power saving of a storage device has been demanded in order to conserve global environment and to reduce the operational cost of a system. Therefore, such a power saving process as to turn off the motor of a disk (a hard disk drive) to which access is not gained for a fixed or more time period is executed on the disk. In this connection, a method of controlling to stop and start each disk by setting in advance conditions such as a minimum drive number indicative of a minimum number of disks to be driven, intervals at which the disks are re-started and others is proposed (see, for example, Japanese Laid-open Patent Publication No. 2008-41050). In addition, a method of predicting the future operating state of each disk on the basis of the operation history of the disk and controlling the operation of the disk so as to reduce the consumption power is proposed (see, for example, Japanese Laid-open Patent Publication No. 2008-250945). Likewise, a method of storing access order and access time for gaining access to each disk as history information and controlling to supply power and stop power supply to the disk on the basis of the history information is also proposed (see, for example, Japanese Laid-open Patent Publication No. 2008-112292)
In order to attain power saving operation of a storage device, it may be necessary to reduce the influence of the power saving operation on the operation of a system concerned by retaining the rapid-responsiveness to access to a disk concerned, in addition to the necessity to reduce the consumption power.
For example, in a storage device that configures an RAID (Redundant Arrays of Inexpensive Disks), power supply to one disk in the RAID configuration is stopped and only disks of the number necessary for access are driven. Owing to the above mentioned operations, power saving operation and rapid-responsiveness to sudden access are secured. As described above, in some cases, the rapid-responsiveness to access is retained by driving a predetermined minimum number of disks. However, a predetermined number of disks are usually in driven states regardless of the operational status of a system and hence such a problem may occur that sufficient effect is not obtained from the viewpoint of consumption power reduction.
On the other hand, in the case that the operation of the disk concerned is controlled by predicting its future operating state on the basis of history information on access to the disk, only the minimum necessary number of disks are driven conforming to the operational status of the system to promote power saving operation. A disk to which access will be possibly gained is started before access is actually gained to the disk by predicting the future possibility of access to the disk and hence the rapid-responsiveness to access may be retained. However, when access to a disk access to which is not predicted has occurred, driving of the disk concerned is started at that moment. Therefore, it sometimes occurs that responding to a host (a host device) is delayed and a response may not be returned to the host side within an access monitoring time (a time period for which access to a disk concerned is monitored).
In addition, in the worst case, there may be a possibility that a storage device is erroneously regarded to be abnormal and hence a path concerned is disconnected. When the path has been once disconnected, it may become impossible to use the path for a fixed time period and hence such a problem may occur that the operation of the system is adversely affected.